Switched-capacitor converters are a class of voltage converters which provide energy transfer and voltage conversion using capacitors. Each leg of a switched-capacitor converter includes a capacitor, and a switch device is connected to each leg for controlling charging of the capacitors. In some implementations, some of the legs also include inductors which makes those legs resonant. In either case, different groups of the converter legs are coupled to different branches of a rectifier at the output. The different groups of converter legs are switched alternately to transfer energy from the input to the output. The rectifier, such as a half-bridge rectifier, rectifies the energy transferred from the capacitors during each switching cycle. The rectified output can be applied directly to a load, or to another converter stage such as a buck converter, a POL (point-of-load) converter, etc.
Initial capacitor voltages and inductor currents at startup may be either zero or different from the steady-state values, causing large spikes and inrush current which place very high stress on the switch devices, especially the switch device closest to the input, and other components. To optimize the size of the capacitors, inductors and switch devices of a switched-capacitor converter, the inrush current should be reduced or ideally prevented. Conventionally, a shunt resistor is placed in series with the switched-capacitor converter input. An external switch is connected in parallel with the shunt resistor to bypass the shunt resistor during post startup operation. The inrush current is reduced, but the approach Is not suitable for pre-charging the capacitors during startup. Hence, a relatively high shunt value is required which makes the startup process very slow. An improved technique is needed for pre-charging one or more capacitors of a switched-capacitor converter,